1. Technical Field
This invention relates to the design of piston and cylinder assemblies and more particularly to improvements that not only reduce the crevice volume of the piston cylinder assembly but enhance sealing contact of the piston rings with reduced friction over an increased temperature range including very low as well as very high temperatures.
2. Discussion of the Prior Art
This invention addresses the problem of differential thermal expansion of a piston relative to its cylinder bore wall causing the piston rings to either provide undue tightness with the bore wall at ultra cold temperatures, such as -20.degree. F., or provide excessive looseness at high temperatures, such as 300.degree. F. The spring tension of conventional piston rings fail to accommodate such wide extremes of temperature. In addition, this invention addresses one or more of five other problems characteristic of current designs for high-temperature piston-cylinder assemblies: (i) excessive crevice volume, (ii) excessive blow-by of fluids, (iii) premature ring fatigue failure, (iv) induced oil combustion, and (v) high cost of machining ring grooves.
Crevice volume (which means the space between the piston and cylinder bore wall, including the groove spaces, up to generally the point of sealing of the bottom compression ring) increases with clearance between the piston crown and bore wall, and increases with groove size. Large crevice volumes are inherent in current piston cylinder designs for commercial automotive internal combustion engines and thus allow for the presence of some unburned fuel and thereby the tendency to increase emissions. Moreover, greater fuel is injected into the combustion chamber at cold start to initiate and sustain combustion; resulting unburned fuel is not readily converted by the catalyst during cold start. Consider also that the design of the piston relative to the cylinder bore is set for the smallest clearance at cold start conditions; thermal expansion of the piston material relative to the bore material, (i.e., aluminum piston to a cast iron bore) will cause the crevice volume to increase at higher temperatures. .
It would be ideal to have a piston that reciprocates within a cylinder bore with no clearance between the piston (crown or skirt) and the bore wall and with little or no friction under all operating conditions. However, to attain durability of the interfacing materials of the piston and cylinder bore wall, materials have been restricted to those which generate undesirable friction, such as iron or steel coated with nickel or chromium for the piston rings, iron or aluminum for the bore walls which sometimes is coated with wear resistant coatings, and iron or aluminum for the piston skirt which sometimes is coated with wear resistance coatings. Attaining zero clearance is even more difficult; the material selection will cause the clearance for pistons in typical cast iron cylinders at top dead center, to vary. For example, aluminum pistons will cause the clearance to vary between 15 microns and 60 microns. The clearance can nearly double under warm operating conditions. Moreover, the bore wall may be scuffed under severe cold start conditions because liquid lubricant may not be present in the ring grooves.
Blow-by allows fluids or combustion gases to leak past piston rings to eventually foul the lubricant on the other side of the rings and create ash within the lubricant itself. Such leakage can be by migration past the backside, front-side or through the split ends of the rings. Gas leakage is usually accompanied by poor oil film scrapping allowing oil to migrate upward into the combustion chamber resulting in contamination by deposits on the combustion chamber walls. Blow-by, particularly front-side leakage, reduces engine compression and robs the engine of its designed power. Conventional ring design is set to create the smallest ring gap at high pressure/high load conditions since the high pressure behind the compression ring will force better sealing contact. But at low load, low speed conditions, gas pressure will not be there and thus the ring gap can get very sloppy. Gas pressure, which acts downwardly on the compression rings, may also freeze the ring against the bottom of the groove or against another ring, induced by high friction; this reduces the ability to maintain proper ring gap with the bore wall. The end gap between the ends of a split piston ring can also increase at high speed allowing an even greater combustion gas leakage.
Premature fatigue failure of a ring is caused by high gas pressure freezing the compression rings to their grooves while the piston slaps against the bore wall jarring and stressing the frozen ring counter to its tension while it is dragged against a non-conforming cylinder wall. Since reciprocating forces change magnitude and direction every 720.degree. F., such stressing constitutes impact loading of the ring; impact loading leads to groove wear, ring instability (commonly referred to as flutter), and eventually ring failure by fatigue.
Induced oil consumption results from a type of peristolic pumping action of oil trapped between the oil ring and the second compression ring (the space adjacent the land between these two rings). On the upward stroke of the piston, such trapped oil is forced back up past the compression rings or behind the compression rings into the combustion chamber. Oil induced into the combustion chamber leaves a residue or carbon deposit. Induced oil consumption can be significant because oil in the land space is effectively pumped upward during the intake stroke at low speed low load engine conditions. The prior art has experimented with several two-ring designs and three-ring designs to eliminate this problem. However, all of the designs proposed to date have either increased oil consumption while reducing friction or reduced oil consumption by increasing friction with higher ring tension.
Narrow rings (having low height) limit the interfacing contact with the bore wall. Thin or narrow grooves are much more expensive and difficult to machine on a high volume basis. Large grooves with single rings have proved unworthy and inoperable.
The chronological history of piston ring design for automotive applications shows repeated effort to prevent blow-by (loss of compression) noting that the rings did not seal effectively against the bore wall or noting that leakage occurred through the grooves supporting the rings. A variety of wear resistant coatings have been applied to the ring grooves as well as to the exposed circumferential sealing surface of the rings (see nickel coating in U.S. Pat. No. 2,575,214; chromium coating in U.S. Pat. No. 3,095,204; and combination coating of Ni, Co--Mo or Mo in U.S. Pat. No. 3,938,814). Flutter of the rings under reverse loading permitted gas and fluid leakage in spite of such coatings and was hoped to have been overcome by increasing the sealing contact pressure of the split rings in each of the spaced grooves. Unfortunately, such increased contact pressure increases friction which then leads to eventual groove or ring wear in spite of oil lubrication.
Applicants are unaware of any design efforts that successfully increase the sealing pressure of the piston rings without increasing piston friction particularly at wide temperature extremes.